Wireless network dissapears help!

i have a philips SNB6500 wirless base station 11g True turbo router and i have a laptop with an in build wirless b/g card now it works fine normally but somtimes the wireless network just dissapears altogether its really annoying as it dose it normaly when i am downlaoding a file from a p2p or bit torrent and have left it on for a few hours while i go to college or out to whatch a movie when i come back its gone it also happnes when i might be just browing the web but that time my friend would also be over with his laptop and both of us are connected to it i was wondering what is causing it other days i will work fine for hours on end its just happens alot when i leave it downlaoding from a p2p any help or insight would be appreciated